The limited availability of bandwidth is a critical constraint on the capacity of wireless communication systems. To enhance capacity, orthogonal modulation schemes, such as Orthogonal Frequency Division Multiplexing (OFDM), have been developed for the modulation of information onto a carrier and subsequent transmission of the modulated signal.
OFDM is a wideband modulation scheme that divides a frequency bandwidth allocated for a communication session into multiple narrower frequency sub-bands. Each sub-band includes a radio frequency (RF) subcarrier, wherein each subcarrier is mathematically orthogonal to the RF subcarrier included in each of the other subchannels. The orthogonality of the subcarriers allows their individual spectra to overlap without causing interference with the other carriers (i.e., intercarrier interference). The division of a frequency bandwidth into multiple orthogonal sub-bands allows for a modulation scheme capable of a high data rate and very efficient bandwidth usage.
An exemplary OFDM communication system 100 is illustrated in FIG. 1. OFDM communication system 100 includes a transmit side 260 (blocks 102-118) and a receive side 262 (blocks 122-136). On the transmit side 260, a data source 102 sources data, typically a bit stream, to an encoder 104. Encoder 104 applies an error correction code, typically a forward error correction code, to the bit stream and conveys the coded bit stream to a symbol mapper 106. Symbol mapper 106 groups the bit stream into groups of P bits (P-tuples) and then maps each P-tuple to one symbol of M possible symbols to produce a symbol stream, wherein M=2P and each symbol is represented as a point in a constellation of points in a multi-dimensional modulation scheme. Typically a two-dimensional modulation scheme is used, such a multiple phase shift keying (MPSK) or a multiple quadrature amplitude modulation (MQAM) modulation scheme.
Symbol mapper 106 conveys the symbol stream to a serial-to-parallel converter (S/P) 108, such as a demultiplexer. S/P 108 converts the symbol stream from serial to parallel form and applies an output of N parallel symbols, to an orthogonal modulator 110, such as inverse discrete Fourier Transform (IDFT) or an inverse fast Fourier Transform (IFFT) block. Orthogonal modulator 110 modulates each one of N subcarriers by one of the N symbols, wherein each subcarrier is orthogonal to all other subcarriers, to produce N parallel modulated subcarriers. The N modulated subcarriers are then conveyed by orthogonal modulator 110 to a parallel-to-serial (P/S) converter 112, such as a multiplexer, that combines the N modulated subcarriers to produce an output signal 113. P/S converter 112 conveys output signal 113 to a cyclic prefix (C/P) adder 114 that appends a guard band interval, or cyclic prefix, to the signal to produce output signal 115. Signal 115 is then conveyed to an upconverter 116 that upconverts signal 115 from a baseband frequency to a transmit frequency. The upconverted signal is conveyed to a power amplifier (PA) 118 that amplifies the signal and transmits the amplified signal via an antenna.
The receive side 262 of communication system 100 implements the reverse functions with respect to the transmit side 260. A received signal is routed to a low noise amplifier (LNA) 120 that amplifies the received signal and then to a downconverter 122 that downconverts the amplified signal from a transmit frequency to a baseband frequency. The baseband signal is conveyed to a cyclic prefix (C/P) remover 124 that removes a cyclic prefix that had been appended to the signal. C/P remover 124 conveys the cyclic prefix-less signal to S/P converter 126. S/P converter 126 converts the downconverted, prefix-less signal from a serial to a parallel form, outputting N parallel modulated subcarriers. The N parallel modulated subcarriers are conveyed to an orthogonal demodulator 128, such as a discrete Fourier Transform (DFT) or a fast Fourier Transform (FFT), that demodulates the transmitted information based upon the N orthogonal functions used in orthogonal modulator 110. The output of orthogonal demodulator 128 includes N parallel symbols based on the N modulated subcarriers, wherein each symbol of the N parallel symbols is drawn from the M possible symbols of the constellation used on the transmit side 260.
Orthogonal demodulator 128 conveys the N parallel symbols to a P/S converter 132. P/S converter 132 converts the symbols from a parallel to a serial form to produce a symbol stream and conveys the symbol stream to a inverse symbol mapper 132. Inverse symbol mapper 132 produces a bit stream by recovering the P-tuple corresponding to each symbol based on the symbol mapping scheme used by symbol mapper 108. Inverse symbol mapper 132 then conveys the recovered bit stream to a decoder 134. Decoder 134 decodes the bit stream based on the error correction code applied by encoder 104 and conveys the decoded bit stream to a data sink 136.
The key to bandwidth efficiency of an OFDM system is the orthogonality of the subcarriers. In order to maintain carrier orthogonality, OFDM systems append a guard band interval, of time length tg, to each OFDM symbol. Typically, the guard band interval is a copy of the last Tg seconds of the OFDM symbol and is commonly referred to as a “cyclic prefix.” Thus, a transmitted OFDM symbol can generally be viewed as including two intervals, the guard band interval Tg and the OFDM symbol interval Ts, so that the entire period of a transmitted symbol is Ttotal=Tg+Ts. Use of a guard band interval, or cyclic prefix, reduces spectral efficiency since time is consumed repeating part of the information. Therefore, the length of the guard band interval should be limited. However, in order eliminate intersymbol interference (one symbol transmitted in a sub-band interfering with a succeeding symbol transmitted in the same sub-band), the guard band interval must be at least as long as the multipath delay, or fading, introduced into the system by the propagation environment.
In wireless communication systems, multipath delay can be very unpredictable. Multipath delay in such systems is a random phenomenon, and there are instances where the multipath delay introduced to a transmitted signal in a wireless communication system is not shorter than a preassigned length of the cyclic prefix. OFDM systems are designed for a maximum delay, or Tg. Excessive multipath delay in an OFDM system causes a loss of orthogonality among the subcarriers and causes interference among consecutive symbols transmitted in a sub-band, producing an irreducible and unacceptably high error floor, that is, a minimum symbol error rate that cannot be reduced even in a very high signal-to-noise ratio communication.
Therefore, the need exists for a method and apparatus that can reduce error in a transmitted signal and maintain a symbol error rate at an acceptable level under conditions of excessive multipath delay.